JPS5845568A - Hot-wire flowmeter - Google Patents

Abstract

PURPOSE:To maintain the resistance value of a metallic wire at a prescribed value despite a change in ambient temp. by selecting the ratios of the temp. coeffts. of resistors used in a bridge circuit of a hot-wire flowmeter. CONSTITUTION:The potential difference at points (c), (d) of connection with a change in the resistance value of a metallic wire RH contributes to a change in the base voltage of a transistor Q1, and the current supplied from the TR Q1 to a bridge circuit increases or decreases the resistance value of the wire RH so as to maintain the same at a prescribed value. The ratio of the temp. coefft alpha2 of a resistor R2 with respect to the temp. coefft. alpha1 of a resistor R1 is determined at around alpha1/alpha2=(R1+RK)/R1, whereby the influence by the ambient temp. of the resistors R1, R2 is eliminated.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a hot wire flowmeter, and more particularly to a hot wire flowmeter for measuring the intake air amount of an internal combustion engine.

In an internal combustion engine that supplies fuel to the engine through a fuel injection valve, the opening and closing of the injection valve is controlled according to the amount of intake air taken into the engine. An intake air flow meter is provided for measurement. As such an intake air flowmeter, there is a hot wire flowmeter that measures the intake air flow rate by extending a predetermined metal wire in the intake air path, passing a predetermined current through the wire, and measuring the resistance change of the metal wire in response to fluctuations in air flow velocity.

In such a conventional hot wire flowmeter, a first voltage dividing circuit including a first and second resistor connected in series and a second voltage dividing circuit including a metal wire and a third resistor connected in series are connected in parallel. A connected circuit, ie a bridge circuit, is formed. A temperature compensation resistor made of a metal wire is connected in series with the first resistor on the side including the first resistor of the bridge circuit, and the temperature compensation resistor is provided in the intake air path together with the metal wire and the third resistor. hinder The metal wire is heated by electricity during engine operation to become a hot wire, and its resistance value changes as it is cooled to a degree that depends on the intake air flow rate.

The resistance value of the metal wire is detected by the voltage across the third resistor, that is, the output voltage of the second voltage dividing circuit, which changes in proportion to the current flowing through the metal wire, and this voltage is output as a value corresponding to the intake air flow rate. .

Further, a current control circuit is connected to the outputs of the first and second voltage divider circuits, and the current control circuit supplies a current to the bridge circuit according to the potential difference between the divided output voltages of the first and second voltage divider circuits. The bridge circuit operates so that the resistance value of the metal wire always becomes a predetermined value due to the equilibrium condition of the bridge circuit.

However, in such a hot wire flowmeter, since the first and second resistors of the first voltage dividing circuit are formed on the board together with the current control circuit and are provided in the case, temperature changes due to heat generation in the current control circuit may occur. The resistance values of the first and second resistors change due to temperature changes due to heat from the engine or the like around the case.

Therefore, there is a problem in that although the bridge circuit is controlled to be in an equal state, the resistance value of the metal wire is not controlled to be a predetermined value.

SUMMARY OF THE INVENTION Therefore, an object of the present invention is to provide a hot wire flowmeter that controls the resistance value of a metal wire to a predetermined value even if the resistance values of the first and second resistors change depending on the temperature.

The hot wire flowmeter according to the present invention has a first resistance (resistance value: R1)
The temperature coefficient α1 and the resistance value of the second resistor (resistance value: R2) or an approximate value thereof are set.

Embodiments of the present invention will be described in detail below with reference to the drawings.

In the figure, the metal wire RH is preferably made of platinum,
1st. A bridge circuit 1 is formed together with resistors R1 to R3, which are second and third resistors. A temperature compensating resistance pulse is connected in series with the resistance R1 on the side including the resistance R1, and the resistance RK is provided in the intake air passage 2 together with the metal wire RH and the resistance R3. The connection point α between the metal wire RH and the resistor RK of the bridge circuit l is the power supply terminal, and the resistor R2 and R3
The connection point is grounded. Further, a connection point C between the metal wire and the resistor R3 and a connection point d between the resistors R1 and R2 are connected to the differential amplifier circuit 3. The differential amplifier circuit 3 constitutes a current control circuit 4 together with a transistor Q1, and connects to a connection point G.
A voltage proportional to the potential difference between and d is generated at the output end, and the voltage is applied to the base of transistor Q1. A power supply voltage VB is applied to the emitter of the transistor Q1, and the collector is connected to the connection point a.

In the hot wire flowmeter having the above configuration, the potential difference between the connection points e and d due to the change in the resistance value of the metal wire RH is caused by the transistor Q1
By changing the base voltage of the transistor Q, the current supplied from the transistor Q to the bridge circuit I is increased or decreased so as to maintain the resistance value of the metal line RH at a predetermined value.

When such control is performed, the equilibrium condition of the bridge circuit l is satisfied (here, RH, R1, R2, R3 and RK are the metal wires RH
, resistance values of resistors R1, R2, R3 and RH. ). However, in reality, the resistance values of the resistors R1 and R2 change depending on the ambient temperature t°C caused by the heat of the current control circuit 4, etc. According to equation (1), the resistance value of the metal wire ~ when the temperature is t°C is α1: Temperature coefficient α2 of resistance R]: Temperature coefficient α2 of resistance R2. Therefore, in the hot wire flowmeter according to the present invention, the ratio of temperature coefficients αl and α2 is as shown in the following equation.

The reason for this will be explained next. The resistance value of metal wire RH is resistance R
1+ Controlling R2 so that it always has a predetermined value without being affected by the ambient temperature t℃ is the resistance value of the metal wire RH when the ambient temperature is 0℃ and the resistance value of the metal wire RH when the ambient temperature is t℃. must be equal.

In equation (2), when 1=0, equation (1) holds, so equation (2) holds true. Therefore, c(R1α1−α2(R,+RK))=o...
......(4). Since it is not affected by the ambient temperature t'CK, the equation (4) is satisfied. That is, resistance R1
If the ratio of the temperature coefficient α2 of the resistance R2 to the temperature coefficient αl of the resistance R2 is determined to be αl R, 1RK α2RI or an approximate value thereof, the resistance R1゜R
The resistance value of the metal wire RH can be controlled to always be a predetermined value without being affected by the ambient temperature.

As described above, according to the hot wire flowmeter of the present invention, the second resistance R2 with respect to the temperature coefficient α1 of the first resistance R1 of the bridge circuit is
Because of the ratio of the temperature coefficient α2, the resistance value of the metal wire is always controlled to a predetermined value even if the resistance values of the first and second resistors R1 and R2 change depending on the ambient temperature.

[Brief explanation of the drawing]

The figure is a circuit diagram showing an embodiment of the hot wire flowmeter of the present invention. Explanation of symbols of main parts 1... Bridge circuit 2... Intake air path 3...
・Differential amplifier circuit 4...Current control circuit applicant
Japan Electronics Co., Ltd. Representative Patent Attorney Motohiko Fujimura

Claims (1)

[Claims] A first voltage dividing circuit including a first resistor (resistance value: R1) and a second resistor connected in series, and a second voltage dividing circuit including a metal wire and a third resistor provided in the gas passage and connected in series. a circuit, and a temperature compensation resistor (resistance value: RK) of the metal wire provided in the gas passage and connected in series to the first resistor.
and a potential difference detecting means for detecting a potential difference between the divided voltages of the first and second voltage dividing circuits, and a voltage difference detecting means connected in series to one end of the first and second voltage dividing circuits and responsive to an output signal of the potential difference detecting means. and a power supply that supplies a voltage across a series circuit of the first and second voltage dividing circuits and the active element, and the second resistor has a temperature coefficient αl of the first resistor. A hot wire flow meter characterized by having a temperature coefficient α2 of .